Inborn errors in cholesterol (Chol) biosynthesis comprise a group of severe, often lethal, metabolic disorders, of which the Smith-Lemli-Opitz Syndrome (SLOS) the fourth most prevalent recessive human disease is the most well-known. Prior work has established a SLOS rat model, which exhibits a progressive retinal degeneration in which photoreceptors seem more susceptible than RPE cells or Mller glia. While the exact disease mechanism is not yet known, the initial biochemical defect involves inefficient conversion of 7- dehydrocholesterol (7DHC) to Chol, which is catalyzed by DHCR7 (3b-hydroxysterol-D7-reductase, the DHCR7 gene product). We have proposed that this leads to multiple sequelae, including altered gene expression, lipid and protein oxidation, caspase activation, and perturbed membrane structure, which then result in progressive cellular dysfunction and demise. 7DHC is the most oxygen-labile lipid known, and readily forms oxysterols (some of which are extraordinarily toxic to cells), and 7DHC-derived oxysterols tend to be more cytotoxic than are Chol-derived oxysterols. The native environment of the retina (high oxygen tension, iron, and incident light) presents ideal conditions for oxysterol formation. We hypothesize that the demonstrable rise in retina/RPE 7DHC levels with blockade of DHCR7 leads to in situ oxysterol formation, resulting in progressive dysfunction and death of photoreceptors, while RPE and Mller glia are relatively spared. Preliminary data strongly support this hypothesis, which we will test further via three Specific Aims: 1) Using transformed, retina-derived cell lines, we will examine whether 7DHC-derived oxysterols differentially alter gene expression and viability of photoreceptors, vs. RPE and Mller glia, in culture, and also will examine the protective effects of antioxidants; 2) We will examine whether photoreceptors are more sensitive to intravitreally injected 7DHC- vs. Chol-derived oxysterols compared to other retinal cell types in vivo; and 3) We will examine the impact of cell-type specific disruption of cholesterol biosynthesis on retinal structure and function in vivo, selectively knocking out Dhcr7 in rods, RPE, or Mller glia. This will markedly advance our understanding of the SLOS-associated retinal degeneration mechanism, as well as provide new insights into the development of more effective therapeutic interventions for such diseases.